The trees growing on urban streets are subject to a number of hazards, and may appear sickly. Disasters such as the epidemic of 'Dutch elm disease' and the death of trees from the use of de-icing salt are noticeable to city residents and have raised awareness of 'urban forestry'. The most important problems for the vitality of city trees listed in the order of significance are: dryness, salt pollution and nutrient deficiency. In smaller cities, mechanical damage by motorized traffic, construction projects and vandalismpredominate. Other factors considered to be responsible for the deteriorating condition and stunted growth of urban trees are natural gas, acid rain, fumes from industry and traffic, over-shadowing by tall buildings, reduction in the air content of soil by excessive compaction of ground and improper planting and lack of subsequent maintenance.
In British villages, towns and cities there are an average of 17.4 trees per acre, mostly in residential neighbourhoods. In 1993, 60% of British urban trees showed signs of sickness such as leaf loss and discoloration, a worse level than that in many of other 33 countries surveyed. Half of the trees planted in cities die within 5 years from lack of water, nutrient-poor soil and lack of regular attention.
Australia’s cities are dotted with mature eucalypts, now becoming collateral damage from population growth. Mature trees are routinely removed to make way for new suburbs and many consider large and old eucalypts a dangerous nuisance that drop limbs and crack footpaths. It takes at least 100-200 years before a eucalypt reaches ecological maturity. As trees mature, their branches become large and begin to grow horizontally rather than vertically, which is more attractive to many birds as perches and platforms where they can construct a nest. Wildlife also use cavities inside ageing eucalypts. These are formed as the heartwood – the dead wood in the centre – decays. Decaying heartwood in older eucalypts leads to some large branches falling. This is when most eucalypts are removed from urban areas - at the exact point in time when they become more attractive to wildlife. This loss of habitat has a considerable impact on native fauna because around 300 of Australia’s vertebrate species, such as possums, owls, ducks, parrots and bats, have evolved to use these cavities as exclusive places to roost or nest. Mature trees also support high concentrations of food for animals that feed on nectar, such as honeyeaters, or seed, such as parrots. One study found that the number of native birds in an urban park or open space declines by half with the loss of every five mature eucalypts.